1. Area of the Art
The present invention generally relates to the field of manufacturing single Czochralski (CZ) silicon crystals. More particularly, it relates to a method of stabilization of a CZ silicon melt contained in the crucible of a CZ silicon crystal puller.
2. Description of the Prior Art
Czochralski (CZ) silicon crystals are produced by growing single crystal silicon ingots from molten polysilicon contained in a crucible of a CZ silicon crystal puller. However, the molten polysilicon often contains gas bubbles and unmelted polysilicon particles that may stick to the sidewall of the crucible. These bubbles and polysilicon particles can cause the formation of dislocations in crystal growth by aggregating at the liquid-solid interface during the freezing process.
In a conventional process of manufacture, heating elements and heat shields are carefully arranged in a CZ silicon crystal puller to produce temperature convection currents within the molten silicon to establish balanced axial and radial temperature gradients, while the crucible is rotated at a constant, low RPM (rotations per minute) for the purpose of stabilizing the silicon melt. However, the temperature convection currents naturally occurring within the molten silicon and the constant low RPM rotation rate of the crucible are not sufficient to release all gas bubbles and polysilicon particles stuck on the crucible wall.
Several prior art references have disclosed the technique of oscillating the crucible during the ingot growth period. For example, Hyland et al. ("Characterization of Silicon Crystals Grown by the Heat Exchanger Method"; S. Hyland, K. A. Dumas, G. H. Schwuttke, J. A. A. Engelbrecht, D. Leung; Proceedings of the Electrochemical Society; Volume 83-11; Pages 192-199, 1983) disclosed the application of an oscillating crucible technique (OCT) in the heat exchanger method (HEM) for silicon ingot growth. This method utilizes a shaping quartz mold in which molten silicon contained in a crucible solidifies from a single crystal seed.
Katherine A. Dumas et al. ("Silicon Ingot Growth By An Oscillating Crucible Technique"; K. A. Dumas, G. H. Schwuttke, T. Daud; Proc SPIE--Photovoltaics for Solar Energy Applications; Page 121; 1983) also disclosed the application of an OCT for casting silicon crystal from a melt within a crucible, to maintain a sufficiently fast liquid flow at the crystal surface, control localized nucleation, prevent supercooling, and avoid dendritic growth after nucleation.
Daud et al. ("Directional Solidification of Silicon In Carbon Crucibles by an Oscillating Crucible Technique"; T. Daud, K. A. Dumas, G. H. Schwuttke, P. Smetana, K. M. Kim; Conference record of the 16th IEEE Photovoltaic Specialists Conference; Page 63, 1982) further disclosed the application of an OCT for directional solidification of silicon in carbon crucibles.
However, in all of the above cited prior art references, the crucible is oscillated during the growth period of the silicon ingot, not during the stabilization period of the molten silicon--a period preceding the growth period. Further, the oscillation of the crucible is designed and conducted for the purpose of crystal growth, not for the purpose of reducing the bubbles and polysilicon particles contained in the silicon melt.
It is desirable to provide a method of effectively eliminating or at least substantially reducing gas bubbles and polysilicon particles contained in the silicon melt during its stabilization period, to therefore increase the yield and productivity of CZ silicon crystal production.